My little Hummingbird depth sounder is fitted with a water temperature sensor.
I fill a log each time I go out and note the water temperature as well as other details , like broken gear that needs attention..
The warm current that flows northward up the east coast of Japan bumps into the southward flowing just off where I live.
Rich fishing grounds.
They say that it's not worth fishing after the end of August because the southward current takes over and the water cools.
Does it what!
I saw water temps drop from 26 degrees to 15 degrees in two days. Will get down to 12 or 11 which is good for the scallops, oysters and kelp.
But makes for cold winter sailing.

gary

I always enjoy reading about your challenges over there gary, thanks for sharing.
I once jumped in the water in Tassie doing a Hobart delivery, the water temp was 14 deg and I almost cried like a baby.
The Victorians on the boat thought it was hysterical

or this:

I use the temperature going to Lord Howe to tell me when I'm in the EAC. Temp is usually 20-22 then jumps to 26 or so when in the current. You then know you are being pushed south at up to 4 kts.

Sounds amazing. Do you really sail in the depth of Japanese winter? I'm heading to Hokkaido next week for skiing and cannot imagine sailing with the amount of snow you see in the fields.

I live and sail on the Pacific coast of Honshu.
The arctic winds blast down from the north west in Siberia.
They cross the Sea of Japan , pick up moisture and dump it as snow on the west coast and on into the mountains.
And of course into Hokkaido
The mountains act as a barrier against the heavy snow.
So on the east coast we get snow on the mountains and all of the fun that comes with it without the extremely heavy snows on the west.
I will add a pic of Mt Zao in spring. This is on our side. The side that doesn't get so much snow!

As for sailing, from now we will choose our days. 8 to 10 knots,with cloudless days.
If the sun is out even at 6 or degrees it's nice.
Especially when you look back to the west and can see the snow covered mountains.